To study the function of zona pellucida specific antigens in the initial spermegg interaction and their possible role in fertility control, cynomolgus monkeys were immunized with porcine zonae pellucidae. To assess the levels of antibody titers, a simple and rapid, solid-phase RIA was developed utilizing 125I protein A. The study demonstrated that monkeys can be immunized against heterologous zonae and rendered infertile. Indications are that the antibody can not reach the zonae of large antral follicles since several monkeys became pregnant just at the time circulating titers of antizona antibodies reached maximal. Instead, the antibody may interfere with progression of primordial and small primary follicles to antral follicles. Infertility was reversible in some of the monkeys, but they had still births to otherwise normal babies. The oocyte population was considerably reduced by atresia by the end of 18 months of the study.